1. Field of the Invention
This invention relates to an analog amplifier construction, preferably a multi-stage amplifying device with high linearity. The amplifier topology has applications in high dynamic range amplifier systems for low and medium frequencies.
2. Description of the Related Art
Traditionally, global feedback has been used to reduce non-linearities for amplifiers consisting of several cascaded amplifier stages. FIG. 1 shows such a global feedback amplifier. Each of the gain stages G1, G2 and G3 will have a phase shift, and to maintain stability when closing the feedback loop it is necessary to include a compensation capacitor Cc to give the amplifier a sufficient phase margin to avoid oscillations when driving reactive loads. For amplifiers driving highly capacitive or inductive loads such as loudspeakers or motors, the phase compensation has to be increased to have a sufficient stability margin for the worst case reactive load. Such a frequency compensation method will reduce the amount of loop gain available for reduction of non-linearities at high frequencies. A loop gain plot is shown in FIG. 2, showing that there is not much loop gain LG2 for reducing non-linearities at the frequency F2. The loop gain at F2 is about 10 dB, giving about 1/100 of reduction of non-linearities compared to the reduction at the frequency F1. At low frequencies such as F1 the loop gain is substantial with about 50 dB of loop gain to correct non-linearities.
Global feedback amplifiers do also have three other types of drawbacks:
The connected output cable feeding the connected load will act as an antenna, and will pick up radio frequency disturbances. These RF disturbances will reach the output of the amplifier, and will also reach the negative input terminal of the input stage of the amplifier through the feedback network. The RFsignal will be rectified by the transistors in the input stage and will shift bias of the amplifying stages. This will increase distortion of the amplifier circuit.
Current kick-back from the connected reactive loads of the amplifier will reach the output of the amplifier. This kick-back will also reach the input stage through the feedback network, and will disturb the operation of the input stage.
The time delay of the amplifying stages will result in a delayed feedback signal to the negative input terminal of the input stage. To avoid generating transient intermodulation in the input stage, the input signal to the amplifier must be bandwidth limited according to the forward path time delay properties of the amplifying stages. In an amplifier driving highly reactive loads, the frequency compensation necessary to insure stable operation will result in larger delays and phase shifts through the amplifier, and hence the bandwidth limiting of the input signal must be increased. Because of this, the overall bandwidth of such a global feedback amplifier will be restricted.
To solve the problem of global feedback amplifiers it has previously been suggested to make a cascaded amplifier as shown in FIG. 3.
This topology solves some problems, but will also introduce some new problems.
The bandwidth of such an amplifier can be increased because there is no need for limiting the bandwidth of the input signal to avoid transient intermodulation in the input stage. The input stage will not be disturbed by RF signals present on the cable connected to the output of the amplifier, as the RF signals will be on the output transistors only. The kick-back from the connected reactive load will not reach the input stage.
The main disadvantage by using the cascaded stages in FIG. 3 is that the non-linearities of the different amplifying stages will be too large to make a high dynamic range linear amplifier for both low and high frequencies when amplifying both weak and strong signals. Often the linearity when amplifying weak signals can be sufficient, but the linearity will decrease when amplifying large signals, because of the voltage and current dependent non-linearities of the semiconductors used in the amplifying stages at high voltage swings and high output current. The output impedance of the output stage will not be sufficiently low to make the frequency response flat when connected to a load with frequency dependent impedance, such as a loudspeaker. This will give frequency dependent amplitude deviations from flat frequency response, and hence a coloring of the amplifier's sonic signature dependent on the connected loudspeaker load.
Error correction networks for single amplifier stages have been proposed earlier, see for example U.S. Pat. No. 5,179,352. However, U.S. Pat. No. 5,179,352 uses complex circuits like differential amplifiers to perform continuous error correction. The U.S. Pat. No. 5,179,352 circuit will only work for non-inverting voltage gains near 1. There is however a need of improved amplifiers with error correction for inverting and non-inverting gain stages with up to 60 dB of gain.